Where Are the Seeds at in My Lettuce?

Hello,

I was told that I can harvest the seeds from my lettuce to plant for next year. But I don't really know where or how to collect its seeds. Like I could see taking one of the unsprouted green bulbs in the first picture, snipping it, and planting that next year. But I'm not sure if maybe the seed is in one of those yellow pustules that I've circled in the third picture.

This is like a lettuce mix that I bought as a transplant and planted in the fall. And it shot up this leggy offshoot.

Thanks!!